Hydrocharitaceae

The Hydrocharitaceae (scientific name Hydrocharitaceae) are a family of perennial aquatic herbs, submerged or floating, distributed throughout the world, most of which are freshwater, although they are also There are marine genres. The family is recognized by modern classification systems such as the APG III classification system (2009) and the APWeb (2001 onwards). It was formerly the only family in the order Hydrocharitales, but in the above classification systems it is included in the order Alismatales. Its leaves are sometimes stalked but usually undifferentiated. The inflorescence often has two fused (sometimes free) bracts at the base. The ovary is inferior, often with laminar or more or less strongly intrusive parietal placentation. The stigma lobes are bifid. The family is morphologically very heterogeneous, being subdivided into 4 subfamilies: Hydrocharitoideae, Stratiotoideae, Anacharidoideae and Hydrilloideae. Pollination mechanisms also vary greatly within the family.

Description

Theoretical Introduction in Descriptive Terminology of Plants

Aquatic herbs, completely submerged to partly emergent, and rooted in the substratum or floating and without fixation, in marine or freshwater habitats, often rhizomatous, tissues more or less aerenchymatous.

Thick-walled, unicellular hairs with extensions of the epidermis into "prickle" structures along margins or veins.

Alternate and spiral leaves, opposite or whorled, along the stem or in a basal rosette, simple, entire or serrated, sometimes with a well-developed blade, with parallel or palmate venation, or evident only on the vein medium, sheathers at the base. No stipules. Small scales present at the node inside the leaf sheath.

Determinate inflorescences, sometimes reduced to a solitary, axillary flower, below it 2 bracts, often conate.

Bisexual or unisexual flowers (plants then monoecious or dioecious), usually radial, with perianth differentiated into calyx and corolla.

Sepals 3, separate, valvate.

Petals 3, separate, usually white, imbricated, sometimes missing.

Stamens 1, 2, 3 or numerous, filaments separated to conate. Pollen usually without apertures, in Thalassia and Halophila united in thread-like chains.

Carpels usually 3-6, conate, ovary inferior, with ovules scattered on the surface of the locules, the placenta often more or less deeply intrusive, styles often divided, appearing twice the number of carpels, stigmas elongated and papillosus

Ovules numerous (or solitary and basal).

Nectar often secreted from staminodia.

Fleshy fruit, it can be a berry or a capsule that opens irregularly or valvately. Embryo sometimes curved. Without endosperm.

Ecology

Widely distributed, but more common in tropical and subtropical regions, in freshwater (most genera) or marine (Enhalus, Halophila, Thalassia) habitats.

The family possesses an interesting variety of pollination mechanisms. Many species in Egeria, Limnobium, Stratiotes, and Blyxa have showy flowers that are above the water surface and are pollinated by various nectar-gathering insects. In Vallisneria, Enhalus and Lagarosiphon, the male flowers detach and float on the surface of the water, while they come into contact with the female flowers. In Elodea, the anthers of the male flowers may burst, scattering pollen grains on the surface of the water, the male flowers themselves are sometimes detached from the plant and float on the surface of the water to the stigma.. In Hydrilla pollen transport can occur by wind or water. Finally, in Thalassia and Halophila pollination occurs underwater.

Cross pollination or self pollination may occur.

The fleshy fruits ripen underwater.

The fruits or seeds are dispersed by water or animals.

Vegetative reproduction by fragmentation of rhizomes is common.

Phylogeny

Theoretical Introduction in Philogenia

Hydrocharitacae, although monophyletic (Dahlgren and Rasmussen 1983, Les et al. 2006) is morphologically heterogeneous, and has been divided into 3 to 5 subfamilies (Dahlgren et al. 1985).

Najas have reduced flowers with a basal and erect ovule, but their placement within Hydrocharitaceae is supported by the anatomy of the seed coat and DNA sequence analyzes (Les 1993, Les and Haynes 1995, Les et al. 2006).

Zannichellia (Zannichelliaceae) probably also belongs here (Les et al. 1997a).

Taxonomy

Theoretical Introduction in Taxonomy

The family was recognized by the APG III (2009), the Linear APG III (2009) assigned it the family number 31. The family had already been recognized by the APG II (2003).

The family was described by Antoine-Laurent de Jussieu and published in Genera Plantarum¡ 67. 1789. The type genus is: Hydrocharis L.

The most represented genera are Ottelia (40 species), Najas (40 species), and Elodea (15 species).

The genera are, according to the Angiosperm Phylogeny Website: (visited April 2015):

• Anacharis Rich. = Elodea Michx.
• Apalanthe Planch.
• Appertiella C.D.K.Cook & Triest
• Benedictaea Toledo = Ottelia Pers.
• Blyxa Noronha ex Thouars
• Boottia Wall. = Ottelia Pers.
• Ageria Planch.
• Elodea Michx.
• Enhalus Rich.
• Inhydrates Ridl.
• Halophila Thouars
• Hydrilla Rich.
• Hydrocharis L.
• Hydromystria G.Mey. = Limnobium Rich.
• Lagarosiphon Harv.
• Limnobium Rich.
• Maidenia Rendle
• Najas L.
• Nechamandra Planch.
• Oligolobos Gagnep.
• Ottelia Pers.
• Stratiotes L.
• Thalassia Banks ex C.Koenig
• Udora Nutt. = Elodea Michx.
• Vallisneria L.
• Xystrolobus Gagnep. = Ottelia Pers.

Synonyms, according to APWeb: Enhalaceae Nakai, Halophilaceae J. Agardh, Hydrillaceae Prantl, Najadaceae Jussieu, nom. cons., Thalassiaceae Nakai, Vallisneriaceae Link.

Economic importance

Many genera, such as Hydrilla, Egeria, Elodea, Vallisneria, and Limnobium, are used as aquarium plants.

Species of Elodea, Hydrilla, and Lagarosiphon are noxious aquatic weeds.

References Cited